August 1, 1998 / Vol. 23, No. 15 / OPTICS LETTERS 1235
s
.
n
a
th
u
ie
e
excludes most conventional piezoelectric and pyroelec-
tric conta
detection
surement
temperat
delivery i
In this
of implem
niques th
use of an
thermally
film sensor.
photoacousti
tic response
6
we demonstr
photoacousti
taneously w
tothermal r
absorber.
measurand-induced phase shifts by operation of the
ting and de-
ls in a targetThe use of this type of probe for making
c measurements
5
and its ultrasonic acous-
were reported previously. In this Letter
ate the ability of the system to make both
c and photothermal measurements simul-
ith high sensitivity and evaluate its pho-
esponse by using a nonscattering liquid
Fig. 1. Schematic of sensor head for genera
tecting photoacoustic and photothermal signact transducer configurations. Radiometric
can be used for same-side photothermal mea-
s
1
but has the disadvantages of relatively low
ure resolution s,0.1
±
Cd and, if optical f iber
s required, the need for use of special f ibers.
Letter we describe an alternative method
enting photoacoustic and photothermal tech-
at can overcome the above limitations by the
all-optical probe based on an acoustically and
sensitive transparent Fabry–Perot polymerOptical fiber photoacou
P. C. Beard, F. Pe´renne`s, E
Department of Medical Physics and Bioengineeri
11-20 Capper Street, L
Received M
We describe the operation of an all-optical probe
photoacoustic and photothermal investigative techniqu
The probe is based on a transparent, acoustically and
mounted at the end of an optical fiber. We demonstra
photothermal measurements simultaneously and eval
liquid target of known and adjustable absorption coeffic
and 6 3 10
23 ±
C, respectively, obtained over a 25-MHz m
Optical Society of America
OCIS codes: 110.5120, 350.5340, 000.1435.
Pulsed photoacoustic and photothermal techniques
are investigative methods in which short subablation-
threshold excitation laser pulses are absorbed in a tar-
get absorber, producing both acoustic (thermoelastic)
and thermal waves. These waves act as carriers of
information relating to the optical, acoustic, and ther-
mal properties of the target absorber and can be used
to describe its constituents and structure. Applica-
tions include the nondestructive testing of materials
and structures
1
and the characterization of biologi-
cal media.
2,3
Although photoacoustic and photother-
mal techniques provide an inherently powerful means
of characterizing a target, their practical implementa-
tion can present difficulties. This is particularly true
when it is required that (i) the generation and de-
tection of the photoacoustic or photothermal signals
take place on the same side of the target for reasons
of limited access and (ii) the acoustic–thermal detec-
tor be placed close to or in contact with the target to
avoid diffraction-induced distortion in the photoacous-
tic signal and low photothermal sensitivity owing to
the rapid attenuation of thermal waves. The acous-
tic–thermal detector therefore has to be transparent
(e.g., as in the method used in Ref. 4) so that it does not
obstruct the excitation laser beam; this requirement0146-9592/98/151235-03$15.00/0tic–photothermal probe
Draguioti, and T. N. Mills
g, University College London, Shropshire House,
ondon WC1E 6JA, UK
y 29, 1998
at provides an alternative means of implementing
es, particularly those used in biomedical applications.
thermally sensitive Fabry–Perot polymer film sensor
te the ability of the system to make photoacoustic and
ate its photothermal response, using a nonscattering
nt. The acoustic and thermal noise f loors were 2 kPa
asurement bandwidth and 30 signal averages.  1998
A schematic of the sensor head is shown in Fig. 1.
A multimode optical f iber with a transparent Fabry–
Perot polymer f ilm sensor mounted at its distal
end is placed in contact with the target absorber.
Nanosecond, submillijoule optical pulses at a suitable
wavelength are launched into the fiber, transmitted
through the sensor, and absorbed in the target, pro-
ducing thermal waves with a typical duration of the
order of a few hundred milliseconds. In addition,
rapid thermal expansion occurs, generating ultrasonic
thermoelastic waves with a typical duration of several
hundred nanoseconds. Both thermal and thermo-
elastic waves are detected by the sensor at the tip of
the f iber. The sensor itself comprises a transparent
50-mm-thick polyethylene terepthalate (PET) film
acting as a low-f inesse Fabry–Perot interferome-
ter, which is illuminated by light launched into the
fiber from a cw low-power laser source. An incident
thermal or thermoelastic wave changes the optical
thickness of the film and hence the optical phase
difference between the Fresnel ref lections from the
two sides of the film. This change produces a corre-
sponding intensity modulation in the light ref lected
from the sensing film, which is then detected by a
photodiode. Linear operation is achieved for smallabsorber.
 1998 Optical Society of America

1236 OPTICS LETTERS / Vol. 23, No. 15 / August 1, 1998interferometer at quadrature. The water cavity
(Fig. 1) provides an acoustic and thermal impedance
match to the fiber side of the sensing film. It also
ensures that the Fresnel ref lection coefficients on
both sides of the film are the same (assuming that
the refractive index of the target is close to that of
water), resulting in high fringe visibility for optimum
sensitivity.
Excitation laser pulses were provided by a frequency-
doubled Q-switched Nd
:
YAG laser operating at
532 nm. The 633-nm cw output of a 6-mW He–Ne
laser was used to interrogate the sensor. Both laser
wavelengths were launched into a 10-m length of
380-mm-core all-silica optical fiber acting as the down-
lead to the sensor head, which was placed in direct
contact with the target absorber. Details of the sensor
head design and the method of attaining quadrature
based on exploiting variations in thickness across the
area of the sensor f ilm are described in Ref. 6. The
signal-modulated cw 633-nm light ref lected back from
sensing film was detected by a 25-MHz silicon p–i–n
photodiode, the output of which was displayed and
signal averaged by a 500-MHz digitizing oscilloscope.
To avoid obscuring the photoacoustic signal we re-
moved repetitive optical and electrical noise from the
Q-switched laser by subtracting a reference waveform
containing only the noise.
5
The target absorber used
was a nonscattering ink of known absorption coeff i-
cient m
a
.
7
This ink was diluted with an acidic buffer
(Tris, pH 3) to yield a range of solutions of different
values of m
a
. The incident f luence was 0.27 mJymm
2
.
We obtained the acoustic system sensitivity by
comparing the sensor output with a calibrated
25-MHz polyvinyl f luoride membrane hydrophone;
it was 140 mVyMPa, with an acoustic noise f loor of
2 kPa over a 25-MHz measurement bandwidth and
30 averages. We established the dc thermal system
sensitivity by placing the sensor head in a water bath
and recording the sensor output as the temperature
was varied. A calibrated thermocouple placed im-
mediately adjacent to the sensor head was used as a
reference. The dc thermal system’s sensitivity was
32 mVy
±
C, with a thermal noise f loor of 6.3 3 10
23 ±
C,
also over a 25-MHz measurement bandwidth and 30
averages. By varying the temperature over 25
±
C
we could observe a maximum and a minimum of the
interferometer transfer function (indicating a phase
shift of p rad), giving a temperature (phase) sensitivity
of 0.13 rady
±
C.
Figure 2 demonstrates the dual sensing ability of the
system, showing the photoacoustic and photothermal
signals generated in an ink–Tris solution of absorp-
tion coeff icient m
a
­ 70 cm
21
over several time scales.
Figure 2(a) shows the short-duration (400-ns) pho-
toacoustic signals; it is assumed that the sensor
gives an accurate representation of them because of
its wideband (20-MHz) uniform acoustic frequency
response.
6
The initial thermoelastic wave X (peak
acoustic pressure, 80 kPa) is that generated immedi-
ately adjacent to the sensing film by the absorption of
the Q-switched laser pulse, whereas the second, much
smaller amplitude thermoelastic wave Y is the time-
delayed ref lection of X from the tip of the optical f iber.The step decrease immediately following the initial
thermoelastic wave X is due to the initial heating of
the target by the laser pulse and can be regarded as
the onset of the rising edge of the thermal wave. This
slow increase in the thermal signal can be seen more
clearly in Fig. 2(b), which shows Fig. 2(a) over a longer
time scale. The full photothermal signal is shown in
Fig. 2(c): the thermoelastic waves cannot be seen on
this time basis because of the relatively long sampling
interval of the digitizing oscilloscope.
To examine the thermal response of the sensor we
generated photothermal signals in ink–Tris solutions
of different values of m
a
, as shown in Fig. 3. These
signals are inverted compared with those in Fig. 2,
so increasing temperature is now represented by a
positive sensor output. For m
a
­ 70 cm
21
, the peak
temperature increase of the photothermal signal, as
determined from the measured dc thermal sensitivity,
is 0.23
±
C. Assuming that all the laser energy is
absorbed and converted to heat and taking the density
and the specific heat capacity of the ink absorber to
be those of water, the calculated peak temperature
(using simple calorimetry) is a factor of 2 higher at
0.45
±
C. The discrepancy arises from the fact that the
dc thermal sensitivity was obtained under steady-state
conditions in which the temperature across the sensor
film was constant. The photothermal wave, however,
has a short wavelength, resulting in a significant
temperature gradient across the film; i.e., the sensor
film is thermally thick. Thus, because the sensor
output represents the integral of the temperature
distribution across the f ilm, the use of the dc thermal
sensitivity calibration causes the peak temperature to
be underestimated.
Fig. 2. Sensor output in response to photoacoustic and
photothermal signals generated in an ink–Tris absorber
sm
a
­ 70 cm
21
d over different time scales. (a) Photoacous-
tic signals. X is the initial thermoelastic wave generated
immediately adjacent to the sensing f ilm, and Y is the re-
f lection of X from the f iber tip. (b) Increased time scale of
(a), showing the photoacoustic signal and onset of the rise
of the thermal wave. (c) Expanded time scale, showing
the complete thermal wave. Fluence, 0.27 mJymm
2
; pulse
duration, 5 ns; repetition rate, 16 Hz; signals averaged
over 30 shots.